Specific Spatial Frequency Research Articles

Retinal Sensitivity in Macular Subfields and Their Association with Contrast Sensitivity in Early and Intermediate Age-Related Macular Degeneration.

The objective of this study was to evaluate retinal sensitivity in subfields and its association with the novel quantitative contrast sensitivity function (qCSF) in patients with early age-related macular degeneration (eAMD), in patients with intermediate AMD (iAMD), and in healthy controls. In this prospective longitudinal study, retinal sensitivity of a customized 24-point grid was assessed by microperimetry Macular Integrity Assessment (MAIA, CenterVue, Padova, Italy) and divided into different subfields. The Multiple Contrast Vision Meter (Adaptive Sensory Technology, San Diego, CA, USA) was used for qCSF testing. Linear models were used to test the association of functional metrics with variables of interest. 92 study eyes from 92 participants were analyzed (13 eAMD, 31 iAMD, and 48 controls). Microperimetry subfield comparison showed significant differences (p < 0.0001) in the control group between superior and inferior hemifield as well as between central and peripheral subfields. For eAMD, significant differences were found between central and peripheral subfields (p < 0.001) and specific subfields (p < 0.05) and finally for iAMD between specific quadrants (p < 0.05) and specific squares (p < 0.05). Significant associations of retinal sensitivity with qCSF metrics were found for the area underneath the logarithmic contrast sensitivity function, contrast acuity and for the contrast sensitivity at specific spatial frequencies. This study showed significant differences in the evaluated retinal sensitivity subfields, providing localized natural history data for retinal sensitivity in healthy controls and patients with eAMD and iAMD.

A comprehensive assessment of a prototype high ratio antiscatter grid in interventional cardiology using experimental measurements and Monte Carlo simulations

Objective. To assess the performance of a new antiscatter grid design in interventional cardiology for image quality improvement and dose reduction using experimental measurements and Monte Carlo (MC) simulation. Approach. Experimental measurements were performed on an angiography system, using a multi-layered tissue simulating composite phantom made from of poly(methyl methacrylate), aluminium and expanded polystyrene (2/0.2/0.7 cm). The total phantom thickness ranged from 20.3 cm to 40.6 cm. Four conditions were compared; (A) 105 cm source-image receptor distance (SID) without grid, (B i) 105 cm SID with grid ratio (r) and strip density (N) (r15N80), (B ii) 120 cm SID without grid, and (Biii) 120 cm SID with high ratio grid (r29N80). The system efficiency (η), defined by the signal-to-noise ratio, was compared from the B conditions against case A. These conditions were also simulated with MC techniques, allowing additional phantom compositions to be explored. Weighted image quality improvement factor (ηw (u)) was studied experimentally at a specific spatial frequency due to the SID change. Images were simulated with an anthropomorphic chest phantom for the different conditions, and the system efficiency was compared for the different anatomical regions. Main results. Good agreement was found between the η and ηw (u) methods using both measured and simulated data, with average relative differences between 2%–11%. Case B iii provided higher η values compared to A, and B i for thicknesses larger than 20.3 cm. In addition, case B iii also provided higher η values for high attenuating areas in the anthropomorphic phantom, such as behind the spine. Significance. The new antiscatter grid design provided higher system efficiency compared to the standard grid for the parameters explored in this work.

Subjective Affective Responses to Natural Scenes Require Understanding, Not Spatial Frequency Bands.

It is debated whether emotional processing and response depend on semantic identification or are preferentially tied to specific information in natural scenes, such as global features or local details. The present study aimed to further examine the relationship between scene understanding and affective response while manipulating visual content. To this end, we presented affective and neutral natural scenes which were progressively band-filtered to contain global features (low spatial frequencies) or local details (high spatial frequencies) and assessed both affective response and scene understanding. We observed that, if scene content was correctly reported, subjective ratings of arousal and valence were modulated by the affective content of the scene, and this modulation was similar across spatial frequency bands. On the other hand, no affective modulation of subjective ratings was observed if picture content was not correctly reported. The present results indicate that subjective affective response requires content understanding, and it is not tied to a specific spatial frequency range.

Spatially-coded Fourier ptychography: flexible and detachable coded thin films for quantitative phase imaging with uniform phase transfer characteristics.

Fourier ptychography (FP) is an enabling imaging technique that produces high-resolution complex-valued images with extended field coverages. However, when FP images a phase object with any specific spatial frequency, the captured images contain only constant values, rendering the recovery of the corresponding linear phase ramp impossible. This challenge is not unique to FP but also affects other common microscopy techniques -- a rather counterintuitive outcome given their widespread use in phase imaging. The underlying issue originates from the non-uniform phase transfer characteristic inherent in microscope systems, which impedes the conversion of object wavefields into discernible intensity variations. To address this challenge, we present spatially-coded Fourier ptychography (scFP), a new method that synergizes FP with spatial-domain coded detection for true quantitative phase imaging. In scFP, a flexible and detachable coded thin film is attached atop the image sensor in a regular FP setup. The spatial modulation of this thin film ensures a uniform phase response across the entire synthetic bandwidth. It improves reconstruction quality and corrects refractive index underestimation issues prevalent in conventional FP and related tomographic implementations. The inclusion of the coded thin film further adds a new dimension of measurement diversity in the spatial domain. The development of scFP is expected to catalyse new research directions and applications for phase imaging, emphasizing the need for true quantitative accuracy with uniform frequency response.

Effects of spatial frequency cross-adaptation on the visual number sense.

When observing a simple visual scene such as an array of dots, observers can easily and automatically extract their number. How does our visual system accomplish this? We investigate the role of specific spatial frequencies to the encoding of number through cross-adaptation. In two experiments, observers were peripherally adapted to six randomly generated sinusoidal gratings varying from relatively low-spatial frequency (M = 0.44 c/deg) to relatively high-spatial frequency (M = 5.88 c/deg). Subsequently, observers judged which side of the screen had a higher number of dots. We found a strong number-adaptation effect to low-spatial frequency gratings (i.e., participants significantly underestimated the number of dots on the adapted side) but a significantly reduced adaptation effect for high-spatial frequency gratings. Various control conditions demonstrate that these effects are not due to a generic response bias for the adapted side, nor moderated by dot size or spacing effects. In a third experiment, we observed no cross-adaptation for centrally presented gratings. Our results show that observers' peripheral number perception can be adapted even with stimuli lacking any numeric or segmented object information and that low spatial frequencies adapt peripheral number perception more than high ones. Together, our results are consistent with recent number perception models that suggest a key role for spatial frequency in the extraction of number from the visual signal (e.g., Paul, Ackooij, Ten Cate, & Harvey, 2022), but additionally suggest that some spatial frequencies - especially in the low range and in the periphery - may be weighted more by the visual system when estimating number. We argue that the cross-adaptation paradigm is also a useful methodology for discovering the primitives of visual number encoding.

The effects of spatial frequency on the decoding of emotional facial expressions.

The accurate identification of emotion is critical to effectively navigating our social lives. However, it is not clear how distinct types of visual information afford the accurate perception of others' emotion states. Here, we sought to examine the influence of different spatial frequency visual information on emotion categorization, and whether distinctive emotional dimensions (valence and arousal) are differentially influenced by specific spatial frequency content. Across one pilot and two experiments (N = 603), we tested whether emotional facial expressions that vary in valence, arousal, and functional significance differ in accuracy of categorization as a function of low, intact, and high spatial frequency band information. Overall, we found a general decrease in the breadth of emotional expressions for filtered images but did not see a decrease in accuracy of categorization for the positive emotion, joy. Together, these results suggest that spatial frequency information influences perception of emotional expressions that differ in valence and arousal. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Spatial frequency domain imaging for the assessment of scleroderma skin involvement.

Systemic sclerosis (SSc) is an autoimmune disease characterized by the widespread deposition of excess collagen in the skin and internal organs, as well as vascular dysfunction. The current standard of care technique used to quantify the extent of skin fibrosis in SSc patients is the modified Rodnan skin score (mRSS), which is an assessment of skin thickness based on clinical palpation. Despite being considered the gold standard, mRSS testing requires a trained physician and suffers from high inter-observer variability. In this study, we evaluated the use of spatial frequency domain imaging (SFDI) as a more quantitative and reliable method for assessing skin fibrosis in SSc patients. SFDI is a wide-field and non-contact imaging technique that utilizes spatially modulated light to generate a map of optical properties in biological tissue. The SFDI data were collected at six measurement sites (left and right forearms, hands, and fingers) of eight control subjects and ten SSc patients. mRSS were assessed by a physician, and skin biopsies were collected from subject's forearms and used to assess for markers of skin fibrosis. Our results indicate that SFDI is sensitive to skin changes even at an early stage, as we found a significant difference in the measured optical scattering () between healthy controls and SSc patients with a local mRSS score of zero (no appreciable skin fibrosis by gold standard). Furthermore, we found a strong correlation between the diffuse reflectance (Rd) at a spatial frequency of 0.2 mm-1 and the total mRSS between all subjects (Spearman correlation coefficient = -0.73, p-value < 0.0028), as well as high correlation with histology results. The healthy volunteer results show excellent inter- and intra-observer reliability (ICC > 0.8). Our results suggest that the measurement of tissue and Rd at specific spatial frequencies and wavelengths can provide an objective and quantitative assessment of skin involvement in SSc patients, which could greatly improve the accuracy and efficiency of monitoring disease progression and evaluating drug efficacy.

Photonics Large-Survey Telescope Internal Motion Metrology System

Large survey telescopes are vital for mapping dark energy and dark matter in the deep universe. This study presents a fiber-linked internal motion metrology system that aligns the mirrors and large lenses in the telescopes to enhance alignment accuracy by improving the image quality at a lower weight, volume, power, and cost. The internal motion system comprises a photonic laser beam projector capable of projecting multiple Gaussian beams onto the detector of the telescope. The specific spatial frequency aberration component is determined by combining Gaussian beam location and the geometry model of the telescope. Furthermore, integrating the proposed system with the curvature-sensing wavefront system enables more precise alignment and camera sensing. In the experimental tests, the location precision was within 10 μm, and the rotation precision improved to 5 arcsecs, fulfilling the alignment and motion monitoring requirements of large survey telescopes. The results of this study can be used as a reference to improve the performance of closed-loop bandwidth systems and active camera optics.

Biases in the spectral amplitude distribution of a natural scene modulate horizontal size perception.

Visual perception is a complex process that involves the analysis of different spatial and temporal features of the visual environment. One critical aspect of this process is adaptation, which allows the visual system to adjust its sensitivity to specific features based on the context of the environment. Numerous theories highlight the significance of the visual scene and its spectral properties in perceptual and adaptation mechanisms. For example, size perception is known to be influenced by the spatial frequency content of the visual scene. Nonetheless, several inquiries still exist, including how specific spectral properties of the scene play a role in size perception and adaptation mechanisms. In this study, we explore aftereffects on size perception following adaptation to a natural scene with a biased spectral amplitude distribution. Twenty participants had to manually estimate the horizontal size of a projected rectangle after adaptation to three visually biased conditions: vertical-biased, non-biased, and horizontal-biased. Size adaptation aftereffects were quantified by comparing the perceptual responses from the non-biased condition with the vertical- and horizontal-biased conditions. We found size perception shifts which were contingent upon the specific orientation and spatial frequency distribution inherent in the amplitude spectra of the adaptation stimuli. Particularly, adaptation to vertical-biased produced a horizontal enlargement, while adaptation to horizontal-biased generated a decrease in the horizontal size perception of the rectangle. On average, size perception was modulated by 5-6%. These findings provide supporting evidence for the hypothesis that the neural mechanisms responsible for processing spatial frequency channels are involved in the encoding and perception of size information. The implications for neural mechanisms underlying spatial frequency and size information encoding are discussed.

A biologically oriented algorithm for spatial sound segregation.

Listening in an acoustically cluttered scene remains a difficult task for both machines and hearing-impaired listeners. Normal-hearing listeners accomplish this task with relative ease by segregating the scene into its constituent sound sources, then selecting and attending to a target source. An assistive listening device that mimics the biological mechanisms underlying this behavior may provide an effective solution for those with difficulty listening in acoustically cluttered environments (e.g., a cocktail party). Here, we present a binaural sound segregation algorithm based on a hierarchical network model of the auditory system. In the algorithm, binaural sound inputs first drive populations of neurons tuned to specific spatial locations and frequencies. The spiking response of neurons in the output layer are then reconstructed into audible waveforms via a novel reconstruction method. We evaluate the performance of the algorithm with a speech-on-speech intelligibility task in normal-hearing listeners. This two-microphone-input algorithm is shown to provide listeners with perceptual benefit similar to that of a 16-microphone acoustic beamformer. These results demonstrate the promise of this biologically inspired algorithm for enhancing selective listening in challenging multi-talker scenes.

Phase Demodulation Based on K-Space With High Sensitivity for Interferometric Fiber Sensor

This study firstly proposes and experimentally demonstrates a phase demodulation method with high sensitivity for interferometric fiber sensors (IFSs) on the basis of the fast Fourier transform of a wavenumber domain. The phase information of IFSs triggered by changes in environmental parameters is obtained by calculating the initial phase variation of a specific Fourier-transformed spatial frequency. Theoretically, phase sensitivity can be improved by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> times when the optical path difference (OPD) of a spatial frequency peak is increased by a multiple ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> times) of the OPD of other spatial frequency domain peaks. To verify the method experimentally, this study designed a large laterally offset spliced sensor formed by common single-mode fibers on the basis of mode interference. The sensing characteristics of temperature and strain in each set of two-beam interference are analyzed simultaneously by calculating the phase sensitivities of the frequency domain peaks. Furthermore, the highest reported temperature and strain sensitivities of 0.0795 rad/°C and −0.0088 rad/μϵ, respectively, with good repeatability are realized.

Spatial Frequency Bias in Convolutional Generative Adversarial Networks

Understanding the capability of Generative Adversarial Networks (GANs) in learning the full spectrum of spatial frequencies, that is, beyond the low-frequency dominant spectrum of natural images, is critical for assessing the reliability of GAN-generated data in any detail-sensitive application. In this work, we show that the ability of convolutional GANs to learn an image distribution depends on the spatial frequency of the underlying carrier signal, that is, they have a bias against learning high spatial frequencies. Our findings are consistent with the recent observations of high-frequency artifacts in GAN-generated images, but further suggest that such artifacts are the consequence of an underlying bias. We also provide a theoretical explanation for this bias as the manifestation of linear dependencies present in the spectrum of filters of a typical generative Convolutional Neural Network (CNN). Finally, by proposing a proof-of-concept method that can effectively manipulate this bias towards other spatial frequencies, we show that the bias is not fixed and can be exploited to explicitly direct computational resources towards any specific spatial frequency of interest in a dataset, with minimal computational overhead.

Imageless Shape Detection Using a Millimeter-Wave Dynamic Antenna Array and Noise Illumination

We present a dynamic millimeter-wave antenna array with rotational dynamics to detect shapes in a scene without image formation. The rotationally dynamic two-element receiving array is able to capture responses in the spatial Fourier (spatial frequency) domain corresponding to sharp edges in the spatial domain. By implementing a dynamic rotation of 180°, a ring-shaped filter is obtained in the spatial frequency domain. Sharp edges in a scene manifest broad spatial–spectral responses that are localized at specific spatial frequency angles and extend radially outward. The ring filter is orthogonal to these strong responses and thus captures them as a function of spatial frequency angle. Shapes with multiple sharp edges produce responses at angles orthogonal to the edge direction, thereby supporting the ability to classify objects based only on the strength and angular location of the spatial frequency responses. Accurate representation of the spatial Fourier domain requires the scattered fields to be spatially and temporally incoherent, and thus, we transmit noise signals from multiple separate noise transmitters, thereby illuminating the scene with incoherent radiation. The dynamic antenna array thus obtains a high signal-to-noise ratio on the received signals, allowing the use of traditional, low-gain millimeter-wave hardware. We evaluate the concept experimentally with a 38-GHz, two-element dynamic antenna array. We demonstrate the detection of spatial frequency signals via rotational dynamics for several reflecting targets of different shapes. The results demonstrate good agreement with simulations and further support the feasibility of detecting shapes in a scene without image formation.

Asymptotics for 1D Klein-Gordon Equations with Variable Coefficient Quadratic Nonlinearities

We initiate the study of the asymptotic behavior of small solutions to one-dimensional Klein-Gordon equations with variable coefficient quadratic nonlinearities. The main discovery in this work is a striking resonant interaction between specific spatial frequencies of the variable coefficient and the temporal oscillations of the solutions. In the resonant case a novel type of modified scattering behavior occurs that exhibits a logarithmic slow-down of the decay rate along certain rays. In the non-resonant case we introduce a new variable coefficient quadratic normal form and establish sharp decay estimates and asymptotics in the presence of a critically dispersing constant coefficient cubic nonlinearity. The Klein-Gordon models considered in this paper are motivated by the study of the asymptotic stability of kink solutions to classical nonlinear scalar field equations on the real line.

Occipital cortex is modulated by transsaccadic changes in spatial frequency: an fMRI study

Previous neuroimaging studies have shown that inferior parietal and ventral occipital cortex are involved in the transsaccadic processing of visual object orientation. Here, we investigated whether the same areas are also involved in transsaccadic processing of a different feature, namely, spatial frequency. We employed a functional magnetic resonance imaging paradigm where participants briefly viewed a grating stimulus with a specific spatial frequency that later reappeared with the same or different frequency, after a saccade or continuous fixation. First, using a whole-brain Saccade > Fixation contrast, we localized two frontal (left precentral sulcus and right medial superior frontal gyrus), four parietal (bilateral superior parietal lobule and precuneus), and four occipital (bilateral cuneus and lingual gyri) regions. Whereas the frontoparietal sites showed task specificity, the occipital sites were also modulated in a saccade control task. Only occipital cortex showed transsaccadic feature modulations, with significant repetition enhancement in right cuneus. These observations (parietal task specificity, occipital enhancement, right lateralization) are consistent with previous transsaccadic studies. However, the specific regions differed (ventrolateral for orientation, dorsomedial for spatial frequency). Overall, this study supports a general role for occipital and parietal cortex in transsaccadic vision, with a specific role for cuneus in spatial frequency processing.

Fearful face scrambles increase early visual sensory processing in the absence of face information

Emotional facial expressions elicit distinct increased early electrophysiological responses. Many studies report even emotional modulations of very early sensory processing at about 80 and 100ms after stimulus presentation, indexed by the P1. These early effects are often interpreted to index differential responses to biologically relevant expressions. Since specific spatial frequencies differ between fearful and neutral expressions, it has recently been suggested that these early modulations are substantially driven by such low-level visual differences. However, it remains unclear whether similar P1 effects are also observed in experiments in which no recognizable face information is presented at all. This study investigated this question and explored also whether any effects depend on colour information and attentional conditions. Participants (N=20) performed a continuous perceptual task of low or high difficultly and were presented with task-irrelevant black/white and colour images of fearful and neutral faces, rendered unrecognizable by doing Fourier phase transformation. ERP findings revealed increased P1 amplitudes for fearful scrambles regardless of experimental conditions. Taken together, our findings show early emotional effects in the absence of any facial expression. Specific low-level frequency information seems to increase P1 amplitudes which thus might have implications for the interpretation of very early sensory emotional expression effects.

Band-limited double-phase method for enhancing image sharpness in complex modulated computer-generated holograms.

Herein, we propose a band-limited double-phase method to improve the quality of reconstructed images encoded by double-phase holograms (DPHs) derived from complex-amplitude light waves. Although the quality of images produced by DPHs was improved compared to that of conventional holographic images, it still suffered from degradation because of the spatial shifting noise generated during the conversion from complex-amplitude holograms to phase-only holograms. The proposed method overcomes this shortcoming by defining a band-limiting function according to the spatial distribution of DPHs in the frequency domain to remove the specific spatial frequency components severely affected by the spatial shifting of DPHs. The sharpness of images reconstructed from band-limited DPHs with appropriate optical filtering showed an improvement of 36.84% in simulations and 51.67% in experiments evaluated by 10-90% intensity variation.

Entrance pupil masks for enhancing moiré-based distance/motion sensing.

Detection of higher-order diffraction patterns when range-finding using moiré images of fork gratings, as previously developed, is key for increased sensitivity. However, higher-order frequencies are inherently lower contrast than the fundamental frequency. Here we explore means of optically increasing contrast of moiré patterns via pupil function design. Two candidate pupil forms that increase performance above diffraction limited capabilities at specific spatial frequencies are considered, and one is chosen to proceed to experimentation. Contrast of the moiré image when using the specially designed pupil is compared to a clear aperture. Both simulation and experimental data are analyzed to show the increased contrast and sensing capabilities of the candidate entrance pupil mask.

Spatial contrast sensitivity in patients with primary open-angle glaucoma and ocular hypertension

To study the characteristics of SCS in patients with POAG, PEG (as well as in the paired eye without glaucoma) and OH. The work was performed in the period from 2011 to July 2017. Analytical, observational case-control study data was analyzed. The study included 1380 patients under dynamic observation; in accordance with inclusion and non-inclusion criteria, 290 patients (540 eyes) with POAG, PEG, OH, as well as healthy people were included in the study. SCS was measured using 'Zebra v 3.0' software developed by the laboratory of clinical physiology of vision named after S.V. Kravkova in collaboration with 'Astroinform SPE' LLC. Patients were shown vertical achromatic (black and white) and colored (red, green, and blue) gratings of specific spatial frequencies (0.5; 1; 2; 4; 8; 16 cycles/degree). Among the patients of the study, 243 (83%) were female and 47 (17%) were male. The average age of all patients at the time of inclusion in the study was 67.75 (62.8; 74.5); 67.84±8.7 years. The study revealed an increase of SCS in patients with OH and their tendency for a slight increase in the contralateral eye with unilateral PEG in comparison with measurements taken from patients of POAG and PEG groups. The revealed increase of SCS is likely to be a consequence of increased or preserved hemodynamic characteristics in the vessels of the eye and orbit in patients with hypertension and in the contralateral eye in unilateral glaucoma, as described in our previous publications. At the same time, generalized changes in the vascular wall in patients with 'PEG-' on the paired eye only predetermine a tendency for an increase of SCS indices.

Differences in early and late pattern-onset visual-evoked potentials between self- reported migraineurs and controls

Striped patterns have been shown to induce strong visual illusions and discomforts to migraineurs in previous literature. Previous research has suggested that these unusual visual symptoms to be linked with the hyperactivity on the visual cortex of migraine sufferers. The present study searched for evidence supporting this hypothesis by comparing the visual evoked potentials (VEPs) elicited by striped patterns of specific spatial frequencies (0.5, 3, and 13 cycles-per-degree) between a group of 29 migraineurs (17 with aura/12 without) and 31 non-migraineurs. In addition, VEPs to the same stripped patterns were compared between non-migraineurs who were classified as hyperexcitable versus non-hyperexcitable using a previously established behavioural pattern glare task. We found that the migraineurs had a significantly increased N2 amplitude for stimuli with 13 cpd gratings but an attenuated late negativity (LN: 400 - 500ms after the stimuli onset) for all the spatial frequencies. Interestingly, non-migraineurs who scored as hyperexcitable appeared to have similar response patterns to the migraineurs, albeit in an attenuated form. We propose that the enhanced N2 could reflect disruption of the balance between parvocellular and magnocellular pathway, which is in support of the cortical hyperexcitation hypothesis in migraineurs. In addition, the attenuation of the late negativity could reflect a top-down feedback mechanism to suppress visual processing of an aversive stimulus.